1. Field of the Invention
This invention relates to an optical module that connects a catheter to a processor in a catheter oximetry system. More specifically, it concerns an optical coupling, that is located within the optical module, for coupling light between a light emitting diode and an optical fiber.
2. Description of the Prior Art
A catheter oximetry system provides accurate, continuous, real-time measurement of mixed venous oxygen saturation using multiple wavelength reflection spectrophotometry. The color of red blood cells progressively changes from scarlet to purple as the amount of oxygen that the red blood cells are carrying decreases. When light of different selected wavelengths illuminates the blood, the amount of light backscattered, or reflected, at each wavelength depends upon the color, and therefore, oxygen level of the blood. Careful choice of wavelengths allows accurate measurement of oxygenated hemoglobin with minimal interference by other blood characteristics such as temperature, pH, and hematocrit.
Approximately 98% of the oxygen in the blood is chemically combined with hemoglobin in red blood cells. The absorption of red and infrared light is substantially different for oxygenated and deoxygenated hemoglobin, and varies for different wavelengths of light within this red/infrared spectrum. Therefore, the relative amounts of oxygenated hemoglobin and deoxygenated hemoglobin in the blood can be determined by measuring the relative absorption of light at selected wavelengths. The percentage of hemoglobin which is in the oxygenated form is defined as the oxygen saturation of the blood in the equation: ##EQU1## where HbO.sub.2 is the oxygenated hemoglobin concentration and Hb is the deoxygenated hemoglobin concentration.
Earlier attempts at catheter oximetry often resulted in inaccurate measurements, since only two wavelengths of light were utilized. Observed light intensity measurements were related to oxygen saturation by implementing equations which were unable to compensate adequately for significant changes in the observed light intensities which were unrelated to changes in blood oxygen levels.
A catheter oximetry system that is manufactured by the assignee of this patent application, uses three, rather than two, wavelengths. These wavelengths were selected to compensate and correct for changes in light scattering (reflectance) from red blood cell surfaces and from other blood and blood vessel components, thereby providing a direct, accurate and reliable measurement of oxygen saturation. This system has three light-emitting diodes contained in an optical module to provide the light sources for the three selected wavelengths available for performing the oxygen saturation measurements. Light from each of these sources is sequentially transmitted at a rate of 244 pulses of each wavelength per second through a single optical fiber within a catheter to illuminate the blood flowing past the catheter tip. This illuminating light is absorbed, refracted, and reflected by the blood, and a portion of this light is collected by an aperture of a second fiber. This collected light is returned through the catheter to a photodetector in the optical module. The photodetector converts these light signals to electrical signals which are amplified and transmitted to a processor. Using the relative intensities of the signals representing the light levels at the various wavelengths, the processor calculates oxygen saturation. The average oxygen saturation for the preceding five seconds is displayed in a digital readout of the processor display panel and recorded on a strip chart recorder. This computation is updated every second.
In coupling the three light-emitting diodes to a single integrator-optical fiber, each light-emitting diode is optically coupled to a Y-optical fiber, and the three Y-optical fibers are joined to the single integrator-optical fiber at a Y-to-integrator junction. Each light emitting diode to Y-optical fiber coupling has a butt coupling, or a direct coupling, with the elements being joined by epoxy cement. While the elements can be properly aligned before cementing, such alignment can be lost during the cementing operation. These couplings have a further disadvantage once the epoxy cement is set, since they cannot then be readjusted. If one coupling is out of alignment, the entire chip supporting the three couplings must be rejected because of high coupling losses at the misaligned coupling. Also, the optical qualities of the epoxy cement may change over time thereby leading to potential errors in the output readings.
The light emitting diodes used in the prior art devices are of the surface emitter type and emit light in a direction perpendicular to the plane of their p-n junction and likewise perpendicular to their largest geometric faces. It has proven to be difficult to correctly locate a light emitting diode to provide the maximum amount of light energy to a given optical fiber because of the small surface area on the end of the optical fiber accepting the light energy from the light emitting diode.
Proper alignment for optical couplings has often been a problem and has often been given extensive consideration by the prior art. U.S. Pat. No. 4,435,037 shows a light emitting diode mounted on a movable post within a housing in juxtaposition to the end of an optical fiber centered within a channel in a connector housing. The optical fiber is held stationary whereas the post supporting the light emitting diode is moved by a probe inserted through multiple aligning ports until the diode is properly positioned for optimum light coupling into the fiber. U.S. Pat. No. 4,296,998 discloses adjustably positioning an optical fiber relative to a light source to optimize the light output at the end of the fiber and locking the fiber in position within an aperture in a solder body by heating and cooling the solder. U.S. Pat. Nos. 4,135,779 and 4,103,154 disclose the coupling of a pair of optical fibers or optical fiber bundles to a third optical fiber or fiber bundle. U.S. Pat. No. 3,938,895 discloses a method for positioning an optical fiber that involves the passing of light through the optical fiber, detecting the output therefrom through the use of a position-sensitive photodetector, and moving the optical fiber so as to achieve a desired relationship to the position-sensitive photodetector.